Hodgson et al., U.S. Pat. No. 2,948,191, discloses a sphere-based retroreflective sheeting wherein a transparent cover film is anchored in spaced relation to the microspheres at a sufficient distance to provide an air interface thereto.
McKenzie, U.S. Pat. No. 3,190,178, discloses the first commercial form of sphere-based, encapsulated type retroreflective sheeting, which comprises a base sheet in which a dense monolayer of transparent microspheres is partially embedded and partially exposed with a specular reflective metal layer underlying the embedded surfaces of the microspheres; a transparent cover film disposed in spaced relation above the layer of microspheres; and a network of narrow, intersecting polymer based bonds that extend over the surface of the base sheet to adhere the base sheet and cover film together and to divide the space between the base sheet and cover film into hermetically sealed cells or pockets in which the microspheres have an air interface. Such sheeting, which is also known as cellular sheeting, is an "exposed-lens" construction, that is, the microspheres partially protrude from a binder layer and have an air interface. Such a construction gives the sheeting a bright retroreflection, brighter than any other known commercial microsphere-based reflective sheeting.
McGrath, U.S. Pat. No. 4,025,159, teaches an improved version of an encapsulated or cellular sheeting as described above in which improved adhesion to the cover film is obtained through use of appropriate curable binder materials.
In the sheetings taught in both McKenzie and McGrath, there is loss of reflective brightness in the areas occupied by the network of bonds, since in those areas the glass microspheres are swallowed up or embedded in binder material which takes away the needed optical relationships for the microspheres to reflect light.
Wood, U.S. Pat. No. 4,075,049, discloses a method of preparing sphere-based, hermetically sealed retroreflective sheeting having improved resistance to delamination under varying conditions of use. This is accomplished, in part, by imposing a grid pattern on a monolayer of microspheres to provide spacings or essentially clear pathways for flow of resinous material which then bonds or links the overlay film to the sphere binder layer. The brilliance of the sheeting of Wood is reduced because of the nonreflectance of the areas where the microspheres are removed to provide the spacings for the flow of resinous material.